1. Field of the Invention
The present invention relates to a multi-beam optical scanning device, and in particular to a multi-beam optical scanning device which is adapted to perform image formation using a multi-beam laser having plural luminescence parts (luminescence points) as light source means in order to attain high speed and high recording density and is preferable for image forming apparatuses such as laser beam printer, a digital copying machine, and a multi-function printer having an electrophotographic process.
2. Related Background Art
FIG. 27 is a sectional view of a principal part in a main-scanning direction of a conventional multi-beam optical scanning device using light source means having plural luminescence parts.
In the figure, plural light beams, which have been emitted from a multi-beam laser 91 having plural luminescence parts, are converted into substantially parallel light beams by a collimator lens. 92, converged only in a sub-scanning direction by a cylindrical lens 94 having a predetermined refractive power only in the sub-scanning direction, shaped by an aperture stop 93, and focused in a focal line shape extending in a main-scanning direction in the vicinity of a reflective surface (polygon surface) 95a of a rotating polygon mirror 95 serving as a light deflector. Then, the plural light beams, which have been reflected and deflected by the polygon mirror 95 rotating at a constant angular velocity in a direction of arrow 95b in the figure, are respectively condensed in a spot shape on a photosensitive drum surface 97, which serves as a surface to be scanned, by two fθ lens systems 96a and 96b serving as a scanning lens system 96. The light beams are used for scanning at a constant velocity in a direction of arrow 97b in the figure.
In such a multi-beam scanning optical system, if plural luminescence parts A and B are arranged vertically in one line in a sub-scanning direction as shown in FIG. 28, an interval of plural scanning lines in the sub-scanning direction on a photosensitive drum surface becomes much larger compared with recording density. Thus, usually, the plural luminescence parts A and B are arranged to be tilted in a direction corresponding to the sub-scanning direction as shown in FIG. 29. By adjusting a slope angle δ of the luminescence parts A and B, an interval of plural scanning lines in the sub-scanning direction on the photosensitive drum is adjusted accurately according to recording density.
In addition, if plural light beams made incident on a photosensitive drum surface return to a multi-beam laser due to regular reflection of the photosensitive drum surface, oscillation of the multi-beam laser becomes unstable. If the regular reflected light returns to an optical system, reflected light returns to the photosensitive drum surface due to surface reflection of the optical system to cause ghost. Thus, conventionally, as shown in FIG. 30, an angle in a sub-scanning direction formed by the plural light beams to be made incident on the photosensitive drum surface 97 and a normal line of the photosensitive drum surface 97 is set to be a predetermined angle (incident angle) β. Consequently, the regular reflected light on the photosensitive drum surface 97 does not return to the multi-beam laser and the optical system.
If such a structure is adopted in a multi-beam optical scanning device, as shown in FIG. 31, a scanning length of each of the plural scanning lines on the photosensitive drum surface 97 varies, and a deviation in a main-scanning direction is caused in focus positions of respective spots on the photosensitive drum surface 97. As a result, a high-quality image cannot be obtained.
There have been proposed various multi-beam optical scanning devices which solve this problem (e.g., see Japanese Patent Application Laid-Open Nos. H5-333281 and H9-197308). In Japanese Patent Application Laid-Open No. H5-333281, an angle formed by plural light beams in a sub-scanning direction and a normal line of a photosensitive drum surface is set to be equal to or smaller than a predetermined angle, whereby a positional deviation of focusing in the main-scanning direction is reduced. In Japanese Patent Application Laid-Open No. H9-197308, a positional deviation of focusing in a main-scanning direction is cancelled by decentering a focusing optical system and adjusting an amount of the decentering.
However, in the multi-beam optical scanning devices proposed in these laid-open patent applications, both a positional deviation of focusing in the main-scanning direction and a focusing property (spot shape) cannot be satisfied simultaneously.
For example, in Japanese Patent Application Laid-Open No. H5-333281, the positional deviation of focusing in the main-scanning direction is simply reduced and made less conspicuous by setting the angle formed by the plural light beams in the sub-scanning direction and the normal line on the photosensitive drum surface to be equal to or smaller than the predetermined angle. Japanese Patent Application Laid-Open No. H5-333281 does not disclose a fundamental solution against the positional deviation of focusing in the main-scanning direction at all.
In addition, in Japanese Patent Application Laid-Open No. H9-197308, the positional deviation of focusing in the main-scanning direction is cancelled by decentering the focusing optical system and adjusting an amount of decentering. However, if the focusing optical system is used in a decentered state, a focusing spot shape on the photosensitive drum surface tends to be deteriorated. Therefore, it is difficult to attain high image quality and high recording density.